'Classrooms (with OR of) 10 students will be better than those (with OR of) 20 students, because they can get instant feedback from their teachers.'.
I'd like to know which is better, 'with' or 'of'.
I'd go for 'with' in you example. However, wouldn't 'classes' fit the context better than 'classrooms'? With 'classes' either could be used, though in this context, I wouldbe more likely to use 'of'.